The oriented strand board (“OSB”) industry emerged in the late 1970s and soon became a major competitor to the plywood industry. By the year 2000, OSB had already captured more than half of the North American structural panel market. Although many efforts have been made by the OSB industry to improve their products' properties, several OSB properties, such as the strength-to-weight ratio, homogeneous density profile, and dimensional stability, still compare unfavourably with plywood.
OSBs are manufactured from wooden strands combined with a thermosetting resin and consolidated together under heat and pressure in a hot press. Typically, an OSB panel comprises a middle core layer and two outer face layers. In order to develop adhesive bonds between the wooden furnish, it is necessary to produce adequate contact between wood and resin, and raise the temperature to cure the resin. Currently, common commercial pressing operations use a closing time (press platens ramp to the final position) in the range of 25 to 90 seconds. With these durations, the temperature in the middle layer (core) of the OSB is still below the point necessary to soften the wooden furnish and cure the resin in the core layer. Additional time is necessary for the heat to transfer into the core to soften the wooden furnish and cure the resin. Because of this temperature gradient, strands in the surface and bottom layers of the mat that contacted to the hot platens first will be softer than those in the middle layer. When the pressure is applied to the mat, the outside layers compress more than the middle core layer. As a result, commercial OSBs typically have an “M” shape vertical density profile through the vertical direction (higher in the surface and bottom, and lower in the core) as shown in the prior art FIGS. 1 and 2.
FIG. 1 is the vertical density profile of a commercial 23/32″ OSB made with Southern Pine wood furnish. Although the average density of this panel is 44 lb/ft3, the density of the surface and bottom layers can be as high as 57 lb/ft3 but the core layer is only 37 lb/ft3. FIG. 2 is another commercial 23/32″ OSB made with Aspen which average panel density is 35 lb/ft3. The highest density in the outside layer is 45 lb/ft3 and the core layer is only 29 lb/ft3. Both panels have shown the typical “M” shapes in their vertical density profiles—very high density in the surface and bottom but a much lower density in the core. There is currently no known technique to overcome this issue and produce OSBs having a flatter vertical density profile. At the present time, the only way to make the adequate contact for the consolidation is to use a much higher average density to raise the density in the core layer. In addition, when these panels are exposed to water, they would have very high thickness swelling due to the high compression ratio in the surface and bottom layers.
Because of the uneven heating and resin setting nature of a convention of OSB process, the moisture content is regulated such that the moisture content of the outer face layers is significantly higher than the moisture content of the core layer. This is done to promote heat transfer into the core layer by heat conduction.
The average density of conventional OSB is between 35 to 45 lb/ft3 depending on the wood species used. When OSB manufacturers attempt to make a lower density OSB below this range, the first problem they will confront would be a very lower density core with a porous appearance, therefore, causing problems of low strength properties.
Therefore, there is a need in the art for a method of making low density OSB having a relatively homogenous vertical density profile which mitigates the difficulties in the prior art.